As semiconductor geometries continue to shrink, manufacturers have increasingly turned to optical techniques to perform non-destructive inspection and analysis of semiconductor wafers. Techniques of this type, known generally as optical metrology, operate by illuminating a sample with an incident field (typically referred to as a probe beam) and then detecting and analyzing the reflected energy. Ellipsometry and reflectometry are two examples of commonly used optical techniques. For the specific case of ellipsometry, changes in the polarization state of the probe beam are analyzed. Reflectometry is similar, except that changes in intensity are analyzed. Ellipsometry and reflectometry are effective methods for measuring a wide range of attributes including information about thickness, crystallinity, composition and refractive index. The structural details of ellipsometers are more fully described in U.S. Pat. Nos. 6,449,043, 5,910,842 and 5,798,837 each of which are incorporated in this document by reference.
Scatterometry is a specific type of optical metrology that is used when the structural geometry of a subject creates diffraction (optical scattering) of the incoming probe beam. Scatterometry systems analyze diffraction to deduce details of the structures that cause the diffraction to occur. Various optical techniques have been used to perform optical scatterometry. These include broadband spectroscopy (U.S. Pat. Nos. 5,607,800; 5,867,276 and 5,963,329), spectral ellipsometry (U.S. Pat. No. 5,739,909) single-wavelength optical scattering (U.S. Pat. No. 5,889,593), and spectral and single-wavelength beam profile reflectance and beam profile ellipsometry (U.S. Pat. No. 6,429,943). Scatterometry, in these cases generally refers to optical responses in the form of diffraction orders produced by periodic structures, that is, gratings on the wafer. In addition it may be possible to employ any of these measurement technologies, e.g., single-wavelength laser BPR or BPE, to obtain critical dimension (CD) measurements on non-periodic structures, such as isolated lines or isolated vias and mesas. The above cited patents and patent applications, along with PCT Application WO 03/009063, US Application 2002/0158193, US Application 2003/0147086, US Application 2001/0051856 A1, PCT Application WO 01/55669 and PCT Application WO 01/97280 are all incorporated herein by reference.
Normal incidence ellipsometry is a widely used type of optical metrology, both for thin film measurements as well as scatterometry applications. As shown in FIG. 1, an instrument of this type includes an illumination source that typically produces a broadband polychromatic probe beam. The probe beam is directed by one or more refractive or reflective components (represented as a lens in this example) to a beam splitter. The beam splitter redirects the probe beam through a rotating polarizer and objective lens (which is typically composed of a series of reflective and/or refractive components) before reaching a sample under test. The sample reflects the probe beam and a portion of the reflected probe beam is captured by the objective. The reflected probe beam then passes through the rotating polarizer and beam splitter and is directed by one or more refractive or reflective components (represented, again, as a lens) to a spectrometer. The spectrometer converts the probe beam into equivalent signals for analysis by a processor.
For the situation where a grating is being measured by normal-incidence reflectance, the grating is typically oriented so the rulings (grooves) are either parallel or perpendicular to the electric field emerging from the polarizer. These are the two normal modes of the system, where linearly polarized light incident on the grating is reflected as linearly polarized light. The mode where the incident/reflected electric field is parallel to the rulings is conventionally denoted as the transverse magnetic (TM) mode. The other normal mode, where the electric field vector is perpendicular to the rulings, is conventionally denoted the transverse electric (TE) mode. Reflectance measurements using these two modes allow the absolute squares Ra =|ra |2 and Rb=|rb|2 of the complex reflectances ra and rb for electric fields parallel and perpendicular, respectively, to the rulings to be determined.
More general normal-incidence reflectance measurements are also possible. The field vector does not need to be oriented as described above, but can assume any azimuth angle P relative to the rulings. Typically, this angle is varied by rotating the polarizer, although it can be varied by rotating either the polarizer or the grating, either at a fixed angular velocity or as a series of discrete steps. At intermediate angles
      P    ≠          n      ⁢              π        2              ,where n is an integer, the two modes are mixed. This allows the relative amplitude and phase of ra and rb to be determined. Specifically, the intensity of the reflected probe beam as a general function of P can be writtenI(P)=a0+a2 cos(2P)+a4 cos(4P)where the Fourier coefficients a0, a2, and a4 are given by
                                          a            0                    =                                                    3                8                            ⁢                              (                                                                                                                          r                        a                                                                                    2                                    +                                                                                                          r                        b                                                                                    2                                                  )                                      +                                          1                4                            ⁢                              Re                (                                                      r                    a                                    ⁢                                      r                    b                    *                                                  )                                                    ;                                                      a            2                    =                                    1              2                        ⁢                          (                                                                                                              r                      a                                                                            2                                -                                                                                                r                      b                                                                            2                                            )                                      ;                                          a          4                =                                            1              8                        ⁢                          (                                                                                                              r                      a                                                                            2                                +                                                                                                r                      b                                                                            2                                            )                                -                                    1              4                        ⁢                                          Re                ⁡                                  (                                                            r                      a                                        ⁢                                          r                      b                      *                                                        )                                            .                                          This calculation assumes that at P=0 the electric field is parallel to the rulings, and ignores the partial polarization that would be caused by the beam splitter sketched in FIG. 1.
As can be seen, rotating the polarizer generates an additional perspective (Re(rarb*)). However, the rotating polarizer design is unable to measure the complementary quantity, the imaginary component Im(rarb*). In addition, if the polarizer is rotated, partial polarization of the source or polarization sensitivity of the detector can lead to systematic errors that cannot be distinguished from the signal due to the sample. Finally, the configuration provides no intrinsic way of verifying that the rulings are indeed parallel to the electric field vector when P is nominally equal to zero.
The inability to measure Im(rarb*) is also true of non-normally incident ellipsometers that utilize single rotating compensators. Im(rarb*) may be obtained by non-normally incident designs that include modulators in both incident and reflected beams. Designs of this type are obviously more complex (since they include multiple photoelastic-modulators or multiple rotating compensators) and are unable to achieve the small spot sizes available to normal incidence designs. Consequently, there are reasons to believe that measurement accuracy may be improved by other designs that do not suffer these limitations.